The investigation is evaluating Medicare billing protocols, according to the report. UnitedHealth shares sank after The Wall Street Journal reported that the insurer's Medicare billing protocols are under Justice Department investigation.

UnitedHealth Group shares tumbled early Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices.

The U.S. Justice Department has launched an investigation into UnitedHealth's Medicare billing practices in recent months, the Wall Street Journal reported on Friday, citing people familiar with the matter.